A FIRST PRINCIPLES STUDY OF ELECTRIC DOUBLE LAYER CAPACITORS

"Electric double layer capacitors (also called supercapacitors) are a type of energy storage devices with balanced energy and power densities, filling the gap between conventional capacitors and batteries. The energy storage mechanism is through electrostatic interaction between charged electrodes and counter-ions in electrolytes, forming EDLs at interfaces. Graphitic carbon is widely used as an electrode material for EDLCs because it satisfies all the requirements for this application including high porosity, good electric conductivity, electrochemical stability and low cost. It has been long thought that sub-nanometer pores are inactive for charge storage in carbon electrodes because they are inaccessible to solvated ions. This widely-accepted axiom, however, has been challenged by the recent discovery of anomalous increase in the capacitance inside carbon micropores.

This discovery has spurred a great deal of fundamental research, aiming at understanding this intriguing phenomenon. Particularly, theoretical modeling has provided a wealth of microscopic information on the EDLs at carbon electrodes. However, the majority of the theory studies use classical models, omitting the electronic structures of ions and carbon electrodes. In this proposal, the state-of-the-art ab initio molecular dynamics (AIMD) will be employed to investigate the EDLCs, which is the first attempt to offer a full atomistic and dynamical description of the EDLs at carbon electrodes at the electronic structure level. AIMD simulations are computationally demanding, but recent advance in computing algorithm and availability of the UK supercomputing facility (ARCHER) have made it possible to model the EDLs at electrochemical interfaces.

The proposed research will closely collaborate with the experimental group of Prof. Clare Grey at Cambridge. Combining theoretical modelling and analytic techniques (e.g. in-situ NMR), we aim at unraveling the microscopic structures of the EDLs at carbon electrodes, and how the EDL structures affect the capacitances. Another objective is to quantify the electronic charges of the ions adsorbed on the electrode surface using a finite field approach developed in computational solid state physics. This will help obtain a fundamental understanding of the EDL capacitances at the electronic structure level.

Simulating electrochemical interfaces at an atomistic quantum mechanical level is one of the grand challenges in computational science, attracting lots of interests at present due to its importance in energy and environment relates issues. Therefore, the proposed research will be of interest to the wide computational communities. Furthermore, this project will bridge the gap between the fields of solid-liquid interface and solid-solid hetero-junction by connecting the concepts familiar to the individual fields. This conceptual link will have impact on both electrochemists and solid state physicists. Finally, to establish the relation between the EDL structures and capacitances, as intended in this proposal, will provide useful chemical insight into designing more efficient electrode materials for energy storage."